George R. Blumenthal

Rotation curves from baryonic infall : dependence on disk-to-halo ratio, initial angular momentum, and core radius, and comparison with data

Using a simple analytic model of the response of dark matter halos to the dissipative infall of the luminous material to form an exponential disk, we explore the dependence of the final rotation curves on all the relevant parameters: the ratio F≡M b /M of the dissipative baryonic mass M b to the total galaxy mass M including dark matter; the ratio b/R of the disk exponential scale length b to the truncation radius R (beyond which infall can be neglected); the core radius r core of the isothermal halo in the absence of dissipation; and the dimensionless angular momentum parameter λ≡J|E| 1/2 G −1 M −5/2 (where J and E are the total angular momentum and energy of the galaxy)

The largest possible voids

Observational constraints on the isotropy of the microwave background on angular scales greater than 1° seriously limit the possible fluctuations in the gravitational potential in the universe because of the Sachs-Wolfe effect. Consequently, they also provide a limit on the initial density inhomogeneities in the universe. We use these limits to show that the typical voids in the mass distribution in an Ω = 1 universe have limiting diameter D 130 h −1 Mpc

Cosmological velocity-density relation in the quasi-linear regime

We develop practical methods for extracting the mass-density fluctuation field of a cosmological system, δ(x), from the peculiar velocity field smoothed on scales of a few Mpc. The methods are local. They are based on quasi-linear approximations to the gravitational equations of motion of a pressureless fluid and address density fluctuations in the range −0.7≤δ≤4.5. They are tested against exact solutions in special configurations and against cosmological N-body simulations with Ω=1 and with Ω=0.2.

Rayleigh-Taylor Stability of Compressible and Incompressible Radiation-Supported Surfaces and Slabs: Application to QSO Clouds

A new coordinate transformation is developed which greatly simplifies the application of boundary conditions in studies of surface stability. The effect of radiation pressure on Rayleigh-Taylor instabilities is examined for surfaces and slabs in the optically thin (fully compressible treatment) and optically thick (incompressible) limits. The ratio of the wave period to the recombination time has a striking effect on the stability of optically thick structures.In applying these results to QSO emission-line clouds, we find that (1) very optically thin clouds are unstable at their far side (leading edge) but the instability may not grow significantly during the cloud lifetime, (2) clouds which are just optically thick but fully ionized can be stabilized at both surfaces, and (3) optically thick clouds having significant amounts of neutral gas on their leading edges or clouds pushed from the rear by winds or other surface forces are strongly unstable and cannot persist.

Angular clustering properties of gamma-ray bursts and quantitative constraints on their distances

Upper limits on the angular correlation function, w(theta), of gamma-ray bursts are obtained which constrain the sources to be very nearby or very distant. The angular clustering properties of the sources in two recent gamma-ray burst catalogs (Golenetskii et al., 1986 and Atteia et al., 1987) have been derived by determining their two-point w(theta) values, and the correlations of both data sets are consistent with w(theta) = 0. It is shown that if the spatial correlation of gamma-ray bursts resembles that of galaxies or galactic clusters, present gamma-ray burst catalogs must be complete to distances in excess of 100 Mpc, while if gamma-ray bursts are associated with a Galactic disk population, then the upper limits on w(theta) constrain the sampling depth to less than two disk scale heights. 50 refs.

Pregalactic formation of globular clusters in cold dark matter

The pregalactic hypothesis for the formation of globular clusters is reconsidered in the light of Zinns (1985) discovery of a two-component globular population in the Milky Way. For a cold dark matter spectrum, high-sigma fluctuations of 10 to the 5th - 10 to the 6th solar masses are assumed to be the progenitors of the spheroidal population of globular clusters. The mass fraction of globular clusters in galaxies then requires that perturbations above roughly 2.8 sigma survive as globulars, and their observed radii require baryonic collapse factors of order 10. Such an absolute density threshold for globular cluster formation achieves adequate fits to observed cluster radii and densities, the mass fraction of globulars versus Hubble type, the radial density profile of globulars within galaxies, and the globular luminosity function. However, a fixed density threshold criterion for cluster survival lacks convincing physical justification and does not by itself explain the homogeneous metallicities within clusters or the large metallicity variations from cluster to cluster and from galaxy to galaxy. 33 references.

Very large scale structure in an open cosmology of cold dark matter and baryons

The standard cosmology of cold dark matter (CDM) in a flat universe with Gaussian, scale-invariant fluctuations, which is so successful in explaining the major properties of galaxies and their clustering on moderate scales, fails to account for the observed clustering of clusters and streaming velocities on scales of 30-100/h Mpc. Assuming that these observations are being interpreted correctly, an open Friedmann model where CDM and baryons contribute comparably to the mean mass density is considered. In this model the CDM is responsible for galaxy formation, while the presence of the baryons induces a large-scale feature in the spectrum of fluctuations at the precombination Jeans mass, giving rise to structure on very large scales. Using linear analysis and simulations based on the Zeldovich approximation, the resultant cluster-cluster correlation function and mean streaming velocity are found to be compatible with the observations. 71 references.

The efficiency of galaxy formation in clusters

Whether the observed trend in the efficiency of galaxy formation between poor and rich clusters is consistent with CDM-biased galaxy formation scenarios or with other hierarchical clustering scenarios described by a power-law fluctuation spectrum from galaxy to rich cluster scales in a universe with density parameter Omega = 1. A greater statistical correlation is found between galaxy and poor cluster masses than between galaxy and rich cluster masses due to the cluster proximity in filter length. This is the fundamental reason why galaxy formation is more efficient in poor clusters than in rich ones. It is found that the observed trend in the efficiency of galaxy formation and the observed values can be produced within the context of CDM-based galaxy formation scenarios or other scenarios involving flat spectra if X-ray luminous clusters are typically 2-3 sigma objects.

Significant cluster correlations at 30 h-1 Mpc : can standard models be ruled out ?

We compute the cluster autocorrelation function, ξ c (r), of an extended, complete sample of 220 rich (R≥1) Abell clusters with measured redshifts. We correct for projection contamination and pay special attention to estimating the random errors, aiming at rejecting certain theoretical models. While the observed ξ c in the range r<20 h −1 Mpc is in agreement with the predictions of CDM and textures models, we find an excess over both theories in the range 20 h −1 Mpc≤r≤40 h −1 Mpc, significant at beyond the 3 σ level

Lensing by Lyman-Limit Systems: Determining the Mass-to-Gas Ratio

We present a new method for determining the total mass-to-neutral gas ratio H I in Lyman-limit absorption systems. The method exploits the relation between the neutral hydrogen column density and the magnification of background sources due to weak gravitational lensing by these systems. Since weak lensing does not measure the mass directly, this relation must be inverted in a statistical sense to solve for the average H I and its distribution. We demonstrate the feasibility of this approach by using a detailed mock catalog of quasars (sources) and Lyman-limit systems (lenses) and by showing our ability to recover the parameter H I. This mock catalog also allows us to check for systematics in the method and to sketch its limitations. For a universal constant H I and a sample of nQSO quasars, we obtain an unbiased estimate of H I with 95% confidence levels of ±140(105/nQSO)1/2, where the error bars are independent of H I.